Mechanic Couldn't Figure Out Problem

I'm probably not going to be much help, but I have some ideas. Is your mechanic using the Toyota Techstream diagnostics? If not, then some of the Prius-specific fault codes won't be shown. I would suppose that a decent mechanic would have checked most of the easy things like the 12V battery status. It doesn't sound like a weak battery anyway. There is technology behind the operation of the HVAC system in these vehicles. Specifically something called "fuzzy logic" that controls the fan speed for a given outside air temperature and requested inside temperature. Could be a bad sensor for either of these parameters. A random fan might also be a mechanical problem with the fan motor, although this shouldn't be affecting a 2014 yet. It could even be something goofed up in the ventilation doors that direct airflow to the various vents, windshield, dashboard, and floor, while the fan continues to operate normally. Might be a visit to a Toyota service department in your future.

 

try avi at hybrid fix

 

The way the blower motor works on the Prius is through a pulsed signal from the controls on the dash. The faster the pulse, the faster the fan speed. This all takes place from the blower resistor module (which I suspect is where you problem exists) to the fan motor. It’s very simple to check. Three wires: Black power, white ground, blue is pulse signal.

 

I noticed the same thing last week in my new Prius 4 (2018). On one occasion the fan turned off completely. When I tried to figure out what had caused this, I noticed that my temperature setting was set too high which had caused the ac to turn off. As it turned out, a usb cable that I had connected to my smart phone was intermittently hitting the temperature control button next to the 11 inch display. Whenever I used my smartphone (mounted on the adjacent vent) the usb cable would hit the button and randomly change my temperature setting. I found that it doesn't take much to trigger the buttons in the vicinity of 11 inch display. Perhaps you are experiencing something similar to this when you operate your display. To verify this, record the ac temperature setting, then the next time you notice a dramatic change in the fan operation, check the temperature setting again to see if it has changed.

 

it's in LA, avi is a well respected member here. want me to google the address and phone?

 

please keep us posted!

 

I don't mean for this to be insulting, just trying to eliminate the simple stuff: Is your climate control set to Auto? If so, that would modulate the fan speed based on temperature.